<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

    <script>
        var str = 'today is Monday';
        var reg = /Mon/;
        var result = str.match(reg);
        console.log(result);
    </script>
</body>
</html>